Air flow question

If the car is off can air still escape. Like if I were to pump air inside the car would there be a way for it to get out, or will it only escape when the car is on. By car I mean cars, trucks, SUVs, and hybrids.Can the fans that blow air into the car pull in air freely from outside when the car is off, or is there something that stops all air flow when the car is off like a cap or vent door.